The invention relates to an earth-moving device in the form of a tractor-driven scraper having a blade for loosening the soil and an elevator for conveying the loosened soil into the scraper bowl and which has improved means for automatically maintaining the elevator in predetermined sweeping relation with respect to the ground regardless of the depth of cut. In conventional elevating type scrapers the normal running position is determined by a pair of stops, one mounted on the elevator frame and the other on the bowl. The stops are adjustable so as to provide the proper relationship between the conveyor and the undisturbed ground ahead of the blade for an assumed average depth of cut. However, the problem is that the depth of cut, which is under the control of the machine operator, can be varied over rather wide limits. In most prior constructions, when the blade is lowered to obtain a deeper cut, the elevator is correspondingly lowered so that the elevator flights, instead of sweeping the surface of the undisturbed ground at a clearance height, strike the surface with some violence. Conversely, when the blade is adjusted to a higher cutting level, the elevator tends to ride too high, thereby reducing the loading efficiency and retention of the soil.
Where the elevator is positioned too low, the shocks to the elevator structure and its driving system, particularly where the ground is hard, may be so severe as to be of destructive effect. As each flight strikes the ground, the elevator, in effect, tries to "climb up" upon the flight, resulting in a high peak loading, loss of power, and vibration which affects not only the elevator but the whole scraper assembly, with annoyance and fatigue to the operator.
This condition has been so consistently encountered in the past that compromises have been incorporated into the design and operation of the elevator such as driving the elevator at reduced speed and the use of additional, more closely spaced flights, all of which have meant a sacrifice in loading efficiency. Moreover, when the elevator rides too high or too low and does not load efficiently, the loosened soil which piles up ahead of the blade must be constantly pushed by the blade requiring greater tractive effort and the use of greater horsepower.
In an effort to overcome these problems, adjustable stops have been provided to accommodate the elevator height to the particular cutting level being employed, but changing of the stops is burdensome and time-consuming and requires temporary shut down of the machine. Consequently, where the operator must change the cutting height frequently, he usually finds it more practical to tolerate the vibration and other effects than to make frequent stop adjustments.
Efforts have been made in the past to maintain the elevator at a predetermined sweeping height, regardless of depth of cut, by coupling the elevator to the tractor draft frame. For example, in Hyler et al prior U.S. Pat. No. 3,807,063 which issued Apr. 30, 1974, the elevator frame is provided with a stop which engages a cooperating upwardly facing stop on the draft frame. This provides approximate maintenance of the sweeping height of the elevator, considering the fact that the draft frame itself sinks slightly upon tilting the bowl to increase the depth of cut and rises slightly for a shallow cut. Moreover, the patented construction does not permit supervisory control of the maintenance height by the operator or capability of switching to alternate modes of operation. To compensate for changes in level of the draft frame resort has been had to special linkages and servo systems and disclosed in my prior application, Ser. No. 508,360 filed Sept. 23, 1974, now U.S. Pat. No. 3,900,977, and my prior application Ser. No. 510,143 filed Sept. 30, 1974, now U.S. Pat. No. 3,303,623.